The sewage systems of towns and cities customarily discharge into a sewage processing plant. The incoming sewage usually has a water content of about 96% or more and is subjected to aeration, settling, sedimentation and compression procedures resulting in a sludge product of a water content of about 70-80% (hereinafter "concentrated sewage sludge"). The disposal of this concentrated sewage sludge presents an enormous problem. In many instances, the sludge is simply dumped at high sea where it has a tendency to pollute the water and to cause great damage to marine life. According to another proposal, the concentrated sewage sludge is subjected to decaying (putrefaction) procedures in open or closed decomposition spaces resulting in the liberation of methane.
This prior-art procedure is relatively time-consuming and cumbersome and requires plants of large surface area. Further, the putrefied or decomposed sludge obtained in this manner can only conditionally be used as fertilizer since no reliable and safe hygienization takes place under the anaerobic conditions prevailing during the putrefaction or decomposition.
It has also been proposed to add clarification sludge in certain volume percentages to organic waste to be digested, thereby to digest the sludge jointly and simultaneously with the organic waste. However, also according to this procedure only relatively small amounts of sludge can be processed so that, from a practical point of view, the sludge has to be disposed of essentially by deposition.
Concentrated sewage sludge of the kind here discussed contains a flora of predominantly anaerobic microorganism species.
In all prior-art procedures of this kind, care is taken that an excess of oxygen is available in all areas of the reactor where the material to be digested is present.
The closest prior art is, in applicant's opinion, represented by the following:
______________________________________ German Patents 1,139,856, 673,066, 805,044. 946,547. filed June 12, 1969 German 1,929,767, 2,201,789. Offenlegungsschrifts filed filed February 22, January 14, 1967 1972 German Auslegeschrifts 1,592,729, 1,929,767, 1,301,828, filed 1,592,800. filed July 19, January 15, 1966 1966 ______________________________________